In Outlook, the mail "root folder path" is empty by default. For Plesk installations (and many other systems) it needs to be set to "INBOX" (without quotation marks and without leading or trailing slashes) in order to be able to access the folder structure of an IMAP account on your server.
In Outlook, go into your account configuration (normally File > Account > Account settings), select (highlight) the IMAP account you want to edit, then click "change". Type INBOX into the root folder path field, save the settings.